1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for forming a capacitor of a semiconductor device.
2. Description of the Related Art
With the development of technology for manufacturing semiconductor integrated circuits (IC) and extension of their application fields, development of large capacity memory devices is in progress. Large capacity memory devices are highly integrated, thus reducing the area of a memory cell unit and cell capacitance.
Particularly, in a dynamic random access memory (DRAM) device comprised of a capacitor for use as an information storage unit and a switching transistor connected to the capacitor, a reduction in cell capacitance due to a decrease in the area of the memory cell unit lowers the read-out capability of the memory cell and increases soft error. Therefore, the reduction in cell capacitance must be prevented, to achieve high integration of the memory devices.
A capacitor for a semiconductor memory device is comprised of a storage node being a lower electrode, a dielectric film, and a plate node being an upper electrode. In order to obtain a higher capacitance in a restricted area of the capacitor of the semiconductor memory device, research has been conducted in the following three areas: (1) a reduction in thickness of the dielectric film; (2) an increase in effective area of the capacitor; and (3) use of a dielectric film material having a high dielectric constant.
In a method for fabricating a capacitor using a material having a high dielectric constant, an oxide film and a nitride film or a composite film of oxide and nitride films was initially used as the dielectric film, but ongoing efforts have been made to use ditantalum pentoxide (Ta.sub.2 O.sub.5), tita strontium trioxide (SrTiO.sub.3), or tita strontium barium trioxide ((BaSr)TiO.sub.3), each having a high dielectric constant, instead of the above films. Ditantalum pentoxide (Ta.sub.2 O.sub.5) has a dielectric constant of approximately 24, which is six times that of the oxide dielectric film. Thus, ditantalum pentoxide (Ta.sub.2 O.sub.5) has been predicted as the most practical high dielectric film material up to now, even though leakage current has been high due to a deficiency of oxygen within the dielectric film.
The above-described technology has been disclosed in U.S. Pat. No. 5,079,191, "Process for Producing a Semiconductor Device", Jan. 7, 1992.
In the prior art, in order to reduce leakage current in a ditantalum pentoxide (Ta.sub.2 O.sub.5) thin film resulting from a deficiency of oxygen, a dielectric film of ditantalum pentoxide is deposited and then oxidized at a temperature of 600 to 1000 degree C., thus supplying necessary oxygen to the dielectric film and simultaneously increasing the density of the dielectric film.
However, in this method, an oxide film having a low dielectric constant grows below the dielectric film, causing a reduction of the overall capacitance. Also, the high temperature for oxidation causes a decrease in the concentration of impurities on the surface of a lower electrode formed of polysilicon doped with impurities, thus greatly changing Cmin/Cmax.